1. Field of the Invention
The present invention relates to a method of recording a full-color half tone image, and more particularly to a thermal wax transfer type color half tone printing method which provides improvements in preventing color moire and changing of hue caused by a miss registration of colors.
2. Related Art
A color thermal printing method, color ink jet printing method, color electrophotographic recording method and so forth are known as color image recording methods. The color thermal printing method includes a direct color thermal printing method wherein a color thermosensitive recording medium is directly heated to record an image thereon, and a thermal transfer printing method. The thermal transfer printing method includes a thermal wax transfer type printing method and a sublimation type printing method. With the thermal wax transfer type printing method, the back surface of an ink film is heated with a thermal head, and melted or softened ink is transferred to a recording paper. With the sublimation type printing method, dye is sublimated into an image reception layer of the recording paper.
In the thermal wax transfer type printing method, a half tone image is reproduced by using a plurality of heating elements disposed in a main scan direction and by changing the length of the dot recording area in a sub scan direction orthogonal to the main scan direction within a virtual rectangular picture cell. The picture cells have the same size and are arranged in a matrix. Therefore, a plurality of rectangular ink dots form a line extending in the main scan direction in the conventional half tone wax transfer type printing method. Depending upon the pattern included in an original, a plurality of conspicuous parallel lines extending in the main scan direction are included in the printed image, which often appears as moire.
In printing a full-color image by using the half tone wax transfer type printing method, a plurality of ink dots of three primary colors, yellow, magenta and cyan, are superposed one upon another on a recording paper so that the recorded ink dots form a full-color half tone image through a subtractive mixture. Because the size of a picture cell is very small, for example, 125.times.125 .mu.m, and because the recording paper is moved relative to the thermal head in the sub scan direction during the recording, a color registration error, that is, an alignment error between three color separation image frames of a full-color image may easily occur. As a result of such a color registration error, the hue of the full-color image may be quite changed from the original or may have color blur.
To prevent the moire and the change in hue of the image caused by a registration error, a recording method has been suggested, for example, in U.S. patent application Ser. No. 08/024,813 filed on Mar. 14, 1993, wherein the lengths of picture cells in the sub scan direction are changed between cyan and magenta, as conspicuous colors. Also every two cyan picture cells adjacent in the main scan direction are staggered or offset from each other by half the length of the cyan picture cell in the sub scan direction. Also, for the magenta picture cells, every two picture cells adjacent in the main scan direction are staggered or offset from each other by half the length of the magenta picture cell in the sub scan direction
However, if the registration of colors fails in this method, the same color patterns cyclically take place in two adjacent columns extending in the sub scan direction. Such color patterns lower the quality of the image rather than prevent a change of hue or color moire.
As an improvement of the just described method, the applicant of the present invention suggests a method wherein the length of the picture cells of two of the three colors in the sub scan direction is twice as long as the length of the picture cell of the one remaining color in the sub scan direction, and the respective positions for the three colors of picture cells adjacent in the main scan direction are shifted or staggered by half the length of the respective picture cells in the sub scan direction.
However, the improved method still has a problem in that gradation of the one of the three colors is reduced so that it is difficult to reproduce a well balanced gray scale.